1. Field of the Invention
The present invention relates to a manufacturing method of an optical component having a free-form surface, or a molding die of the optical component, for example, suitable for producing mold cores for fθ lenses or free-form surface mirrors for projection TVs.
2. Description of Related Art
Recently, for projection mirrors for projection TVs, mirrors having free-form mirror surface have come to be used. Particularly since rear projection TVs require the thinnest possible optics, free-form surface mirrors are becoming important. To form an optical component having a free-form surface with a metal die, it is necessary to create the metal die, which has the same surface as the desired free-form surface. Generally, in forming a free-form surface without a rotation axis by a cutting work, a fly cutting machine as indicated in FIG. 1 is used (e.g., see Japanese Unexamined Patent Publication Nos. 2000-94270, 2000-298509).
Such a fly cutting machine 1 has a machine platen 11 on which an X-axial slide table 12 and a Z-axial slide table 13 are laid in step-wise. A Y-axial slide table 14 is on the machine platen 11, also. To the Y-axial slide table 14, a tool spindle 15 is attached. To an end part of the tool spindle 15, a tool 16 is attached. The axial slide tables 12, 13, 14 are controlled to move in the respective axial directions. The tool spindle 15 is controlled so that the tool 16 is rotated around a main axis of the tool spindle 15.
The fly cutting machine 1 controls movements of a work 20 placed on the Z-axial slide table 13 in two axial directions; X-axis and Z-axis, in an X-Z plane. Further, the fly cutting machine 1 controls the tool 16 in a single, Y-axial direction while rotating the tool 16 at a predetermined rotation speed. In this way, it is possible to control the tool 16 and the work 20 relatively in three axial directions to carry out a cutting work. In addition to such a fly cutting machine, there is known a machine that employs three-axial direction control in which a work table is controlled in single axial direction and a tool table is controlled in two axial directions.
Generally in a cutting work by the fly cutting machine 1, as illustrated in FIG. 4 of the Japanese Unexamined Patent Publication No. 2000-94270, the tool 16 and the work 20 are moved relatively in a rotating circumferential direction of the tool 16 to cut the work 20 in a line. Then, the tool 16 and the work 20 are moved relatively in a rotating axial direction of the tool 16 to cut the work 20 in the next line. In the course of these procedures, there appears a tool mark in a cut surface by the fly cutting work, as illustrated in, e.g., FIG. 8 of the Japanese Unexamined Patent Publication No. 2000-94270. Additionally, this cutting work is carried out every time in one-way and single direction. This is because the one-way cutting stabilizes a relation between a rotating direction of the tool 16 and a moving direction of the work 20 to produce a uniform characteristic of worked surface.
In recent years, since more small-sized and more high-precision optical components are needed, further improvement of a machining accuracy has been required. Particularly in projection mirrors for projection TVs, even a slight tool mark may cause flare, because its image is magnified. However, in the aforementioned conventional manufacturing method of mold cores, there has been a problem that a surface roughness increases in proportion to cutting length. This sets a practical limit to a forming accuracy of the cutting work to a surface roughness of approximately 0.7 μm in a cut surface of a work, and thus it is difficult to carry out a cutting work at a surface roughness of about 10 nm.
Additionally, in the method, the cutting work is carried out in one direction only as described before. Thus, it takes excessive time to merely move backward. This raises a problem that the whole working time is long and the productivity is low. Further, long working time is apt to change ambient temperature during working. As a result, a work or a machine may be subtly deformed due to a temperature change, so that the worked surface may become wavy. However, a reciprocating work causes variations in characteristics of the worked surfaces in between forward and backward working, leading to another problem that a worked surface cannot be uniform.